The present invention relates to a timing extracting method for use in a PCM (Pulse Code Modulation) transmission system and a circuit therefor and, more particularly, to a method and a circuit for timing extraction applicable to a PCM transmission system capable of transmitting data at the gigabit per second (Gb/s) order.
In parallel with the progress of optical transmission technologies, an ultra high-speed optical transmission system of the type using an optical device operable at a long wavelength band and a single-mode optical fiber is attracting much attention as a promising high-capacity long-haul transmission system. Especially, high-speed and stable optical transmission equipment is indispensable for a broad-band communication network which implements various kinds of services involving data, picture and speech. Such a broad-band communication network has a fundamental transmission system whose transmission capacity is extremely high, e.g. several gigabits per second in a time division multiplex communication system. The network, therefore, needs broad-band high-speed optical transmitting and receiving units. Usually, a receiving unit included in terminal equipment has a timing circuit for defining the center of an eye pattern so that an equalized waveform may be identified correctly. In a PCM regenerative repeater, the timing circuit generally includes a timing extracting circuit which extracts a timing component or information from a received code stream.
For the timing extraction, it has been customary to feed an NRZ (Non-Return to Zero) code or similar received signal having no timing component to the timing extraction circuit via a differentiator or a double-wave rectifier, while feeding an RZ (Return to Zero) code or similar code having a timing component directly to the timing extraction circuit. An element used for a timing extraction is implemented by a SAW (Surface Acoustic Wave) filter, for example. Since timing deviation is one of the critical characteristics of a timing extracting element, the SAW filter is provided with a quality factor Q of the order of 800 in consideration of the temperature characteristic, aging, detuning, and so forth. For details of this kind of timing extracting circuit, a reference may be made to "A 565-Mbit/sec monomode transmission system" reported by D. S. Larner et al. at Optical Fiber Communication (OFC) on Feb. 13, 1985.
The prior art timing extraction circuit using a timing extraction filter in the form of a SAW filter and generating a clock signal whose frequency falls in the gigahertz range directly as stated above has some problems left unsolved. Specifically, the frequency range available with the circuit is limited in relation to the fine machining of the electrodes of the SAW filter, and the yield is low due to the inherent fabrication process. More specifically, in a SAW filter, an excited surface wave has a fundamental frequency f.sub.0 which is determined by the surface wave propagation rate V and the electrode pitch L, i.e. f.sub.0 =V/L. Generally, the surface wave propagation rate V is 3.times.10.sup.8 (meters per second). It follows that for an exciting frequency lying in the gigahertz range a SAW filter having electrodes which are less than 1 micron wide has to be fabricated. A SAW filter used in a 4 Gb/sec optical regenerative repeater, for example, has electrodes each being formed on a quartz substrate and provided with a width of 0.2 microns and a length of 400 microns. It is difficult to fabricate a SAW filter having such an accurate electrode width due to the limits of photoetching, laser machining and similar technologies. This obstructs the realization of a PCM signal receiving unit in the gigabit per second range.